In general, an LCD device has a cross sectional structure as shown in FIG. 1. In the LCD device 1 of FIG. 1, spacers 4 are scattered for forming a predetermined gap between a pair of substrates 2a and 2b having display electrodes 5a and 5b, respectively, on facing inner surfaces of the substrates, and an alignment film 7. The gap between the substrates is filled up with a liquid crystal 3, enclosed by a seal pattern 6. On the outer surfaces of the pair of substrates 2a and 2b, polarizers and the other optical films (not illustrated) are formed at respective optimal positions. The number of polarizers formed on an LCD device are two, one or none depending on the operation mode of the LCD device.
The display of an image is performed by applying voltage to the LCD device 1 and also, in a transmissive type LCD, by irradiating the light of a fluorescent tube or the like from the backside, and, in a reflective type LCD which utilizes ambient light, by disposing a reflective plate at the backside.
The conventional method of manufacturing the above LCD device 1 is described hereinafter referring to the flow chart of FIG. 8. First, at step 800 the substrates 2a and 2b having the display electrodes 5a and 5b, respectively, are cleaned. At step 802, liquid aligning material is then applied by offset printing or the like, then a pre-bake and a full-bake are performed. Thus, an alignment film 7 is formed, then alignment treatment is performed by rubbing, or the like, on the alignment film. In general, cleaning by water is performed after the rubbing (at step 804) for washing off of foreign matter and dirt at step 806.
At step 808 on one of the two substrates, e.g., on 2a, a seal pattern 6 for enclosing liquid crystal 3 is formed by applying sealing material by the method of writing, screen printing, or the like. Then ultraviolet curing type resin (hereinafter referred to as UV resin) is applied by spot-printing using a dispenser or the like outside the display area of the LCD device 1 for temporarily bonding the substrates. In addition, at step 810 on another substrate 2b, spacers 4 of a predetermined size for forming a gap are formed, such as by spraying. At step 812, position alignment of the substrates 2a and 2b is performed in the atmosphere, and, after the alignment, the two substrates are temporarily bonded together. Here, the alignment of the substrates 2a and 2b is performed up to a predetermined accuracy referring to the alignment marks which are optically recognizable and are disposed in advance on the display electrodes of the respective substrates. Then the UV resin for temporarily bonding is cured by irradiating with ultraviolet rays.
At step 814, a gap forming and seal pattern curing process is performed where the substrates 2a and 2b are pressed toward each other by an air-press or the like for obtaining a predetermined thickness of the gap of the LCD device 1, and when the gap becomes the predetermined thickness, the seal pattern 6 is cured. Here, if a heat-curing-type sealing material is used for the seal pattern 6, it is cured by heating with heater lines disposed inside the surface plates of the air-press. If ultraviolet-curing-type sealing material is used for the seal pattern 6, it is cured by irradiating ultraviolet rays through transparent thick surface plates made of glass or an acrylic material. Thus a so-called empty cell is formed.
At step 816, the unused periphery portion of the substrates outside the display area is cut off. At step 818, in the liquid crystal filling process, the cell and a pool of liquid crystal are set in a vacuum chamber, then the air-pressure of the chamber is decreased to approximately 25-90 Pa. The injection openings of the cell are then dipped into the liquid crystal, and the chamber is leaked to atmospheric air pressure, by which the cell is filled up with the liquid crystal 3 by the pressure difference and by a capillary phenomenon. At step 820, the cell filled up with the liquid crystal 3 is taken out from the chamber, and the openings of the cell are sealed with resin or the like, then the extraneous liquid crystal on the cell is washed away. At step 822, the whole cell is annealed for stabilizing the alignment of the liquid crystal 3. With the above processes, the manufacturing of the LCD device 1 is completed.
However, in the above conventional method of manufacturing LCD device 1, it has been difficult to obtain the sufficient precision of the alignment of the substrates and sufficient uniformity of the gap. The reason is that, in the conventional method, the process of substrate alignment and the process of gap forming by pressing are separate. Therefore, the cured ultraviolet resin for temporary bonding after the substrate alignment is sometimes broken by force at the next pressing process, which causes slippage from the aligned positions of the two substrates.
The above problem is more troublesome in larger size substrates, especially, in the case of high resolution thin film transistor (TFT) LCD panel. If the slippage from the aligned position occurs, additional margin becomes necessary for alignment precision between the pixel electrodes formed on one of the substrates and the black matrix formed on another substrate, which causes a serious problem of a low opening ratio.
Also, when ultraviolet-curing-type sealing material is used for the seal pattern, ultraviolet rays are irradiated through the transparent surface plate, which causes the gradual temperature rise of the surface plate by the radiant heat, then, the temperature of the substrate touching the heated surface plate becomes higher than another substrate. If the seal pattern is then cured in the state where the temperature of the respective substrates is different, the non-uniformity of the gap of the LCD device occurs because the attached substrates bend when they are cooled down to the room temperature. The non-uniformity of the gap directly causes non-uniformity of display, which is a serious problem in an LCD device, especially in a large size LCD device.
As described above, in the conventional manufacturing method, it has been difficult to realize the sufficient precision of the substrate alignment and the gap.
In the above, a vacuum injection method, which has been popular for mass-production, is described for filling a cell with liquid crystal. Other than that, there is a dropping method, i.e., a substrate on which a predetermined volume of liquid crystal is dropped in advance and attached with another substrate in a vacuum ambience. Although the order of the liquid crystal filling process is different between the two methods, the problems described above exist in both methods.
The present invention aims to address conventional problems and to provide a method of manufacturing an LCD device for coping with a larger size substrate for a larger size LCD device used for an LCD monitor or the like by which a CRT can be replaced. Further, the present invention aims to provide a manufacturing method for realizing a more precise and uniform gap for a high quality display, and for realizing more precise substrate alignment, thus, providing a brighter display with a larger opening ratio.